Beamrider guidance is a form of Command Line Of Sight (CLOS) guidance system. In this system a beam is projected from a launch position toward a target. The beam is spatially coded so that as the missile looks backward toward the projected beam it can determine its position with respect to the center of the beam or boresight. Therefore, the missile can track the beam to the target.
An advantage of beamrider guidance is the versatility available in beam encoding techniques. For example, three different encoding methods are four quadrant, bar scan, and FM reticle. The four quadrant technique employs four lasers that each provide one quadrant of the total beam. Each laser is pulse position modulated with a code unique to its quadrant. The beam is nutated about the line of sight. A missile on the boresight axis spends an equal amount of time in each quadrant during a nutation cycle, while an off-axis missile receives unequal durations of the quadrant codes corresponding to its position relative to the line of sight. This allows the missile to generate its correctional commands internally.
The bar scan or L beam method of spatial encoding provides more efficient use of laser power because only a vertical bar and a horizontal bar are transmitted. The vertical and horizontal bars are nutated and modulated with an azimuth and elevation code, respectively. When one of the beams crosses the missile receiver, the resulting pulse burst is identified as azimuth or elevation. In this manner the position of the missile relative to boresight can be determined.
The FM reticle method is an encoding technique that requires only one laser. This method employs a reticle composed of alternate transparent and opaque segments. The reticle rotates through the laser beam, projecting the image of the spinning reticle to a missile, resulting in a condition that will generate a fixed modulation frequency at the receiver of a missile located anywhere in the field of view. The reticle also revolves about the guidance field producing an FM signal at the receiver that, when synchronously detected with respect to the revolution angle, provides position information relative to the line of sight.